Trophic Levels and Energy Transfer — The 10% Rule

Q1 — Sample Band 6 response (7 marks), annotated

Trophic efficiency is the percentage of energy transferred from one trophic level to the next, calculated as: Trophic efficiency (%) = (energy at higher level ÷ energy at lower level) × 100. The 10% rule states that on average approximately 10% of energy is transferred between levels, meaning ~90% is lost. [1 — definition and formula]

Three energy loss pathways reduce the available energy at each level: (1) Respiration — organisms continuously break down organic molecules via cellular respiration to produce ATP for movement, growth and reproduction; the majority of chemical energy (60–90%) is released as heat and cannot be used by the next trophic level. (2) Egestion — not all ingested food is digested; cellulose, chitin and bone pass through the gut undigested and are eliminated in faeces (10–30%). (3) Excretion — excess nitrogen from protein metabolism is excreted as urea or uric acid in urine (2–5%). [1.5 — three pathways named and explained; 0.5 per pathway]

Using T1 = 20,000 kJ m⁻² yr⁻¹ and 10% efficiency at each level: T2 = 2,000; T3 = 200; T4 = 20; T5 = 2; T6 = 0.2 kJ m⁻² yr⁻¹. [1 — correct calculation chain reaching T6]

T6 would contain only 0.2 kJ m⁻² yr⁻¹. This is biologically unsustainable because an organism requires energy for basal metabolic rate, movement, thermoregulation, growth and reproduction. Even the smallest predator requires far more than 0.2 kJ per square metre per year — it would need to forage over an enormous territory just to meet its basal metabolic costs, let alone reproduce. No self-sustaining population could form because the density of available prey energy is below the minimum viable threshold. [1 — energy at T6, 1 — explains "biologically unsustainable" in terms of metabolism and population viability]

This is why natural food chains typically stop at 4–5 trophic levels: after 4–5 transfers the energy has been reduced by 10,000-fold or more from the primary producer level, making further trophic levels energetically impossible to sustain. [0.5 — synthesis and link to observation about food chain length; accept as part of earlier point]

Marking criteria.

• 1 mark — Defines trophic efficiency correctly and states the 10% rule formula.
• 1.5 marks — Names and explains all three loss pathways (respiration/heat, egestion/faeces, excretion/urine); 0.5 per pathway.
• 1 mark — Correct calculation from T1 to T6 (T2 = 2,000; T3 = 200; T4 = 20; T5 = 2; T6 = 0.2 kJ).
• 1 mark — States the T6 energy value and identifies it as insufficient to support a viable population.
• 1 mark — Explains "biologically unsustainable" in terms of metabolic requirements — organisms need energy for respiration, movement, growth, reproduction; available energy is below viable threshold.
• 1.5 marks — Overall conclusion linking the energy calculation to the observed rarity of food chains beyond 4–5 levels (accept as part of the explanation above).

Q2 — Sample Band 6 response (8 marks), annotated

Cattle are primary consumers (T2). The 10% rule predicts that approximately 10% of grass energy becomes cattle biomass; in practice, cattle convert only 3% of ingested energy into beef because they are large endotherms with very high metabolic costs. [1 — 10% rule applied to cattle, notes actual efficiency is lower] This means that to produce 1 kg of beef (6,000 kJ), the steer must ingest 6,000 ÷ 0.03 = 200,000 kJ of pasture energy, requiring approximately 13–14 m² of pasture per kg. By contrast, delivering the same dietary energy from wheat (a T1 food) requires only ~0.3 m² per 6,000 kJ (at 20,000 kJ m⁻² yr⁻¹). The T1→T2 energy loss (~97% for cattle) drives a 40–50 fold difference in land requirement. [1 — land area comparison quantified]

Ecological consequence 1: Vegetation structure modification and habitat loss. Grazing removes palatable ground cover plants, reduces vegetation diversity and can lead to dominance by unpalatable or woody weed species. This simplifies habitat structure, reducing the diversity of microhabitats available to ground-nesting birds, reptiles and small mammals — species groups that are highly over-represented in Australia's extinction record. [1 — first consequence correctly identified and explained]

Ecological consequence 2: Soil compaction and degradation. Cattle hooves compact soil, reducing water infiltration and increasing surface run-off and erosion. Compacted soil also has lower porosity, reducing the habitat available to soil invertebrates and affecting decomposer communities. This cascades up the food web by reducing soil nutrient availability to plants. [1 — second consequence correctly identified and explained]

The argument is strong because it correctly identifies trophic inefficiency as the root cause of the land-use disparity. It is well supported by calculation — a 40–50 fold reduction in land need if Australians shifted even partially to plant-based diets would free up millions of km² for potential habitat restoration. [1 — evaluates strength of argument] However, the argument has limitations: it does not account for the economic and social costs of dietary transition, the role of beef production in rural employment, or the fact that some grazing land (arid, rocky, steep) cannot be efficiently used for cropping. Additionally, simply removing cattle does not guarantee native vegetation will recover — active restoration and weed management are also required. [1 — identifies genuine limitations]

Nevertheless, the ecological logic is sound: reducing beef consumption would decrease the area devoted to grazing, decrease habitat disturbance from hoofed animals, and reduce the pressure on native ecosystems — all of which would benefit Australian biodiversity. The strategy is necessary but not sufficient on its own. [1 — reaches a justified, nuanced conclusion]

Marking criteria.

• 1 mark — Applies the 10% rule to beef production, noting cattle's actual efficiency is ~3% and explaining why (large endotherm, high metabolic costs).
• 1 mark — Quantifies the land difference between beef and a T1 crop (wheat), showing the ~40–50 fold difference.
• 1 mark — First ecological consequence of grazing (vegetation modification / habitat structure loss) — identified, explained, linked to biodiversity impact.
• 1 mark — Second ecological consequence of grazing (soil compaction / degradation / salinity / erosion) — identified, explained, linked to ecosystem impact.
• 1 mark — Evaluates the strength of the ecological argument (the 10% rule logic is solid; land reduction would benefit native ecosystems).
• 1 mark — Identifies real limitations of the argument (economic, social, land type suitability for cropping, active restoration needed).
• 2 marks — Reaches a justified, nuanced conclusion (effective ecological strategy; necessary but not sufficient alone; uses precise ecological terminology throughout).

Q3 — Sample Band 6 response (6 marks)

The claim is largely incorrect, containing three biological errors. [1 — clear overall evaluative judgement]

What is partially correct: The observation that decomposers absorb organic material and release heat is correct — decomposers do use cellular respiration during decomposition, releasing heat. However, this does not support the rest of the claim. [0.5 — identifies the one accurate element]

Error 1 — "Exactly 10% is always transferred": The 10% rule is an approximation or average. Actual trophic efficiency ranges from about 5% (endotherms in cold climates with very high metabolic costs) to 20% (ectotherms in warm aquatic environments). "Exactly 10%" is scientifically incorrect. [1 — identifies and corrects error 1]

Error 2 — "Energy is returned and reabsorbed by producers": This conflates energy with matter. Decomposers break down dead organic matter and release inorganic nutrients (nitrogen compounds, phosphates, etc.) that producers can reabsorb. The energy in dead organisms is released as heat during decomposition and cellular respiration — it dissipates to the environment and cannot be recaptured by producers. Energy flows one-way through ecosystems. [1 — identifies and corrects error 2]

Error 3 — "Ecosystems can support unlimited trophic levels": Because energy is not recycled, each successive trophic level has less energy available. After 5–6 transfers the available energy is insufficient to support a viable population, physically limiting food chain length to approximately 4–5 levels. [1 — identifies and corrects error 3]

Defensible reformulation: "The 10% rule is an approximation — trophic efficiency averages approximately 10% but ranges from 5–20%. Decomposers recycle matter (nutrients) but not energy; energy flows one-way from producers to consumers to heat, progressively diminishing at each trophic level. Because energy is lost and not recycled, ecosystems can only support 4–5 trophic levels before available energy becomes too low for viable populations." [0.5 — biologically defensible reformulation distinguishing energy flow from matter cycling]

Marking criteria.

• 1 mark — States a clear evaluative judgement (the claim is largely/substantially incorrect).
• 0.5 mark — Correctly identifies the one accurate element (decomposers do release heat during respiration).
• 1 mark — Identifies and corrects Error 1: 10% is an approximation/average, not an exact fixed value (range 5–20%).
• 1 mark — Identifies and corrects Error 2: decomposers recycle matter (nutrients), not energy; energy is released as heat and is not returned to producers.
• 1 mark — Identifies and corrects Error 3: because energy is not recycled, food chain length is limited to ~4–5 levels.
• 1.5 marks — Reformulates the claim into a biologically defensible statement that correctly distinguishes energy flow (one-way, lost as heat) from matter cycling (recycled by decomposers); explicitly invokes both concepts.